Venetian type blinds have a series of slats hung on ladders which extend from a headrail to a bottomrail. In most venetian blinds a pair of lift cords is provided each having one end attached to the bottomrail and then passing through elongated holes in the slats up to and through the headrail. A cord lock is usually provided in the headrail through which the lift cords pass. The cord lock allows the user to maintain the blind in any desired position from fully raised to fully lowered.
The slats rest on rungs between rails of the ladders. The blind is in an open position when the rungs are horizontal. To close the blind one lifts one rail while allowing the other to either remain in place or be lowered. This raises one end of each rung tilting the slats. Thus, when the blind is closed there is typically tension on one rail while the other rail of the ladder is not in tension.
The rails originally used for venetian blinds consisted of a fabric tape typically from one to two inches in width. The rungs were also made of fabric strips. Conventionally, the lift cords pass through holes in the slats which were aligned between the rails of the ladder. When the blind was closed, the fabric rails covered the holes. Consequently, there was no concern about light passing through the holes when the blind was in a closed position.
Walker in U.S. Pat. No. 2,572,224 discloses slats for a venetian type blind having a tape type ladder system and two lift cords. One lift cord passes through notches on the inside edges at one end of the slats. The second lift cord passes through notches on the outside edge of the other end of the slats. Each lift cord is positioned behind one of the tape-type ladders. Walker does not disclose any lift mechanism inferring that a standard lift mechanism could be used. In a blind made with Walker's slats the lift cords play no role in maintaining the slats in alignment when the blinds are closed. Rather, the tape-type rails of the ladders maintain the slats in alignment.
Very few tape type ladders are used in blinds today. Rather, the art has adopted ladders formed of cord rails having cord-type rungs between them. Typically, the rungs are multiple strands of cord. However, a single strand of cord has also been used. Frequently, a braided cord is used for the ladder rails. The conventional lift cord arrangement in which the lift cord passes through elongated holes in the slats is used. The lift cords are aligned to be between the rails of the ladders which are provided near opposite ends of the blind. When this type of blind is closed light can pass through the holes in the blind slats. This is readily apparent to anyone looking at the closed blind.
In pleated shades efforts have been made to prevent light from passing through the cord holes by making the holes to be a diameter very close to the diameter of the cord. This is not possible in a venetian type blind where the holes must be elongated to allow the slats to tilt properly.
In the venetian blinds which utilize a cord-type ladder the blinds are opened and closed by lifting or lowering one of the ladder rails relative to the other. In most blinds the lateral position of the ladder rails relative to one another remains unchanged at the headrail. Thus, there will be slack at the top of one of the ladder rails and the center of gravity shifts forward or back when the blind is tilted.
Venetian blind slats are conventionally made of aluminum, plastic or wood. Cord route holes are normally punched into the slat one at a time while the length is cut. Wood slats are usually cut to length by saws so that punching or machining of the holes is done one unit at a time. Moreover, punching of holes through wood slats sometimes causes splintering and the areas around the holes are very difficult to paint or seal. After the cord holes have been drilled, lift cords are passed from the bottomrail through the cord holes to the headrail to complete the assembled blind. After the blind has been assembled in this manner it is not possible to remove the slats from the blind without removing the lift cords making it very difficult to clean the slats in the area of the route holes. Thus, it is a major task to clean or paint the slats in this type of blind. Since the lift cords pass through each slat they prevent tight closure when the blind is tilted to a closed position. Further, the route holes weaken the slats dramatically so that bending more easily occurs at the route holes.
There is a need for a venetian type blind having no holes through the slats. The venetian must operate easily and the slats must maintain alignment when the blind is open and closed and most particularly when the blind is raised and lowered.